Low Thyroid Hormone Raises Risk for Type 2 Diabetes

People with prediabetes and low thyroid function were more than twice as likely to progress to full-blown type 2 diabetes compared to those with normal thyroid-hormone levels in a new study1 published September 30 in the journal BMC Medicine.

For the study, researchers from the Erasmus Medical Center in the Netherlands followed 8,492 adults, ages 54 to 74, for nearly eight years. At the start, 1,338 had prediabetes and 7,114 had normal blood sugar levels. After 7.9 years, 798 had developed type 2 diabetes. Overall, low thyroid function—even in the low-normal range—increased risk for diabetes by 13%. But among those with prediabetes, the risk for progressing to type 2 diabetes ranged from 15% for those with normal thyroid function to 35% for those with signs of low function. Even “low normal” thyroid functioning increased risk.

It’s the first large study to track low and low-normal thyroid function and diabetes risk. “There are no other studies addressing the relation between diabetes and thyroid function in the euthyroid range or in individuals with prediabetes,” the researchers note. Lead study author Layal Chaker, MD2 a Research and Curriculum Fellow at the Harvard T.H. Chan School of Public Health and a research fellow in the Department of Endocrinology and Epidemiology at the Erasmus Medical Center told EndocrineWeb that its too soon to recommend thyroid screening tests for people with prediabetes. And while treating low thyroid can help people with type 2 control their blood sugar, it’s not known whether treatment could prevent or delay it. “Our findings suggest that screening in people with prediabetes could be considered for further research,” Dr. Chaker said.

“There are many ways in which low thyroid hormone levels can increase the risk of diabetes,” he said. “First of all, thyroid hormones are crucial for our body’s metabolism and energy expenditure. Low thyroid hormone levels are associated with higher Body Mass Index, which in turn is a risk factor for diabetes.”

But there seems to be a deeper connection. “When we account for [BMI] in our analyses we still see an association of thyroid hormone levels with the risk of diabetes, especially in individuals with glucose levels within the prediabetic range,” he says. “Thyroid hormone is also directly involved in control of insulin secretion and glucose homeostasis. Hypothyroidism is associated with decreased insulin sensitivity and glucose tolerance and treatment of hypothyroidism has shown to improve these effects. The exact mechanism or mechanisms that are involved in the association need to be investigated.”

Participants in the study are all part of the large, on-going Rotterdam Study tracking the development of age-related diseases in residents of Ommoord, a large neighborhood in the Dutch city of Rotterdam. Volunteers’ thyroid status had already been checked by measuring levels of thyroid stimulating hormone (TSH) and thyroxine (also called T4). Elevated TSH and reduced T4 indicate low thyroid function.

If further research confirms a connection, in the future people with prediabetes and low or low-normal thyroid function might be more strongly encouraged to adopt healthy lifestyle changes to prevent prediabetes or even begin diabetes treatment or thyroid treatment sooner, the researchers note.

Endocrinologist Larry A. Distiller, FCP(SA), FRCP, FACE, of the Centre for Diabetes & Endocrinology in South Africa,3 who has studied rates of hypothyroidism in people with type 2 diabetes, agrees that there’s not enough evidence to suggest thyroid screening for people with prediabetes.

“Routine screening for hypothyroidism in those with diabetes or prediabetes is contentious,” Dr. Distiller told EndocrineWeb. “Some guidelines recommend routine screening but most do not. Both hypothyroidism and diabetes are common conditions and common things occur commonly, so it is difficult to assume one leads to the other.” Dr. Distiller added that there’s “no evidence” that treating hypothyroidism would prevent prediabetes from progressing to type 2.